The strategy we employ to prepare organic solvent is to coat the Au NR with a molecule containing a S-H (thiol) group to covalently bind the coating to the surface of the Au NR. Commercially available coatings include thio-polystyrenes, thio-alkanes, and thio-lecithin derivatives. We have also prepared a series of thio-mesogen derivatives at the facility.  The solubility of the Au NR in solution depends on nature of the coating, with longer polymeric coating tending to give better solubility in solution.

Materials List

Thiol (SH) are susceptible to self oxidation converting 2 S-H bonds to a S-S bond, resulting in weaker bonding to the Au NR surface. To prevent this problem, the coating reaction may be run under and inert gas such as nitrogen or argon, described below. That being said,these reaction may be run successfully without using an inert gas if unavailable.

Step 1: Selection of coating material

1. The choice of coating material depends on the application for the material. For Au NR soluble in organic solvent, longer chain polymers are simple to prepare. In this case, the polymer chains act as 'brushes' that prevent the rods from aggregating. Shorting coating can be better for incorporation into polymer networks to avoid micro phase separation, while the phosphatidyl choline is best for biological applications. Coating is an exchange process, with CTAB replaced with the coating material. Both CTAB and the thio-coatings described here are soluble in the solvent tetrahydrofuran,(THF)

Step 2: Coating

1. One question for preparing the coated Au NR is how much coating material is needed to coat the rods From our experiments, we estimate that 10 to 15% of the initial gold is actually converted to Au NR. This corresponds to approximately 1.45E19 gold atoms in the solution of Au NR. If we grossly overestimate the number of Au atoms on the surface of the Au NR as half of this number, this corresponds to ~7.25E18 surface Au atoms, or ~1.25E-5 mol of surface Au atoms. Therefore, for a coating with a molecular weight around 500 g/mol, 6mg of coating would be required to coat the rods. In actuality, this number could be much lower.
2. We have achieved best success when equi-volumes of Au NR solution / THF are used. Use a graduated cylinder to measure the Au NR volume if unknown.
3. Measure out an equal volume of THF to the volume of Au NR solution in a glass vial
4. Weigh out the appropriate amount of thio-coating material. Based on amounts in this procedure, thio-polystyrene (20 mg); thio-alkanes (3 to 4 uL), DPPTE (4mg)thioethanolpho
5. Dissolve the thio-material in the THF
6. Strongly stir the aqueous Au NR solution and add the THF/thio-coating solution dropwise, approximately 1 drop per minute. After all the solution is added, allow to stir for another 30 minutes if coating with polystyrene. If using a non-polymer coating, spin for at least 4 hours before proceeding to the next step.
7. Stop stirring the reaction. At this stage, the Au NR may have precipitated out of solution. This is expected. If the Au NR have not precipitated, centrifuge the solution for about 20 to 30 minutes at high speed.
8. Carefully remove the solvent from the rods using pipette.
9. Residual water on the Au NR will result in the formation of an emulsion when the organic solvent is added to the solution. Gently flow air over vial to remove the water, or place in a 40^oC oven overnight. Temperatures over 60^oC may change the shape of the rods
10. After the rods are dry, add 2 to 5 mL of an an organic solvent such as toluene or chloroform to the vial containing the Au NR. To suspend the rods in solution, gently shake the vial. If the rods are still stuck to the bottom, a vortexer or sonication bath to resuspend the rods in solution The preparation is now complete.
11. Over time, the rods may fall out of solution. When this occurs, sonicate the solution to resuspend

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